This invention relates generally to the sensing of certain conditions associated with the operation of gas furnaces and more specifically to the sensing of condensate and flame roll-out conditions.
Integrated or combined hot surface ignition and fan controls are common in the heating, ventilating and air conditioning (HVAC) industry. Conventional controls employ thermal sensors in the form of bimetal thermostatic sensors for the detection of flame escaping the confines of the combustion chamber in a gas furnace. This flame escaping the combustion chamber is known as "flame roll-out". These thermostatic sensors are normally closed manual reset (or one-shot) type devices. They are located such that when flame escapes the combustion chamber, the thermostatic sensors are heated which causes the normally closed contacts to open. The contacts of the thermostat are wired in series with the gas valve circuit of the control. Thus the gas valve will be de-energized if the flame escapes the combustion chamber. With the advent of multiposition furnaces, as many as four thermostatic sensors must be employed (one for each of the four directions that the escaping flame may rise) to detect the flame roll-out condition. However, the use of four sensors is expensive. Another draw back to the use of thermal sensors is the inherent time delay involved with the heating of the sensors, typically, 30 seconds.
On the other hand, thermostatic sensors provide a desirable characteristic in that all failure modes with the wiring and connections result in safe conditions. In fact, these failures result in an equivalent to the opening of the flame roll-out thermostat's contacts. In the case of one (or both) of the wires connected to the thermostat "broken", the current path for the gas valve is opened (thus the valve is de-energized). If one of the wires to the thermostats is shorted to the chassis of the furnace, power for the gas valve is shorted out and again the gas valve is de-energized. Thus safe operation is achieved in all of the failure modes with thermostat sensors.
Another problem associated with high efficiency gas furnaces presently in use relates to the fact that such furnaces are so efficient that water vapor is condensed from the by products of combustion. This presents additional problems for furnace manufacturers. Condensate must be drained from the vent and the combustion chamber. This is accomplished through a so called collection box which encloses the outlet from the combustion chamber and the inlet to the vent system. The collection box is constructed of a polymer material due to a number of factors such as cost, odd shape and the corrosive nature of condensate. In such a system if the drain becomes clogged, the furnace will begin filling with fluid and its operation will become unsafe. Furnace manufacturers normally solve this problem by adding an extra pressure switch to detect the build up of fluid in the vent (vent pressure changes due to partial blockage and fan restriction).